Plaster molds are particularly useful in the production of ceramic articles such as dishes, vases, toilets and the like. They are used for casting ceramic articles because of gypsum""s favorable water absorption properties. Plaster is also known as calcium sulfate hemihydrate, calcium sulfate semihydrate, calcium sulfate half-hydrate, stucco, or Plaster of Paris. When making a mold, Plaster of Paris is mixed with water to form a slurry. The slurry is then poured over a form, and allowed to hydrate until the setting reaction is complete. As the plaster sets, the hemihydrate form is converted to the dihydrate form through chemical bonding of water to form an interlocking matrix of calcium sulfate dihydrate crystals. As the dihydrate is formed, capillaries are formed in the plaster, giving it the ability to absorb water after the gypsum mold is dry. For use in molds, this ability to absorb water allows the mold to xe2x80x9cwick awayxe2x80x9d water from the clay slip that is poured into the mold to produce xe2x80x9cgreenwarexe2x80x9d which must be fired in a kiln to make a ceramic article.
The use of plaster for preparing molds for the production of shaped articles is well known in the art. For example, U.S. Pat. No. 2,391,855 discloses making permeable plaster bodies and the addition of additives to control expansion of the plaster after the initial set. The addition of potassium salts to plaster that is formed into boards or molds is taught in U.S. Pat. No.4,443,261. Both of these references are hereby incorporated by reference.
In molding pieces of ceramicware, most of the time is used in the process of creating the greenware and then drying the gypsum mold. When the clay slip is poured into the gypsum mold, water is absorbed by the gypsum, allowing the clay particles to deposit on the inner surface of the gypsum mold. Greenware is formed when enough water has been absorbed to allow the cast article to hold its shape and be removed from the mold, although the article is still very fragile at this stage.
The forming process may be accelerated by a process known as medium pressure casting. It includes applying pressure to the clay body while it is still in the mold. Calcined gypsum in the alpha form, which is made by heating gypsum under pressure, produces a mold that is stronger than one formed with beta calcined gypsum and is more suitable for pressure casting. Molds made from alpha gypsum, however, have a disadvantage because the lower porosity of set gypsum formed from alpha calcined gypsum results in longer times to purge and dry the molds.
Gypsum mold materials that absorb water from the clay slip reduce the time required to form greenware. This improves the efficiency of the ceramicware making process by freeing up the mold to be used in casting another article. This is known as xe2x80x9cturn overxe2x80x9d. However, before the gypsum mold can be reused, it must be dried. The water that has been absorbed into the capillaries must be purged from the gypsum mold so that the capillaries will have the capacity to draw sufficient moisture from the next ceramic article. Generally, the gypsum mold is treated with heat or pressure to drive off the absorbed water. Heating or pressurizing the molds to accelerate the drying process also requires costly expenditures of energy. Additionally, the molds begin to deteriorate after many uses because drawing of the water into and out of the capillaries causes erosion. Erosion of the gypsum mold over time, due to gypsum""s natural solubility in water, leads to a loss of detail in the gypsum mold which is then transferred to the cast greenware article. Normally a plaster mold can be used for an average of 80 cycles before erosion makes it unusable.
Many additives are known to change the properties of the plaster used in the mold. Molds made from alpha calcined gypsum are stronger, more dense and less porous compared to those manufactured from beta calcined gypsum. In general, additives that improve the hardness of the gypsum mold usually reduce the porosity of the mold. Strength may also be increased by casting a denser gypsum mold; however, this typically has a negative effect on porosity. Loss of porosity reduces both the ability of the mold to wick moisture away from the clay slip, and also reduces the purge rate of the gypsum mold. Thus, although stronger molds may last longer, they may slow down the production of the greenware articles by increasing both the absorption time and the purge time of the gypsum molds. Molds which are difficult to purge also add to the expense of the process by requiring higher energy usage in the drying process.
Some of these problems have been solved by the use of resin molds in place of gypsum molds for casting of ceramic articles. These molds provide fast turnover of the mold, but the resins used therein are considerably more expensive than gypsumxe2x80x94based compositions. Resin molds must also be made from special machinery. This adds to the cost of the process as the design of the mold needs to be outsourced. In addition, resin molds are only cost effective for large production volumes as the molds are costly to make, store, and maintain.
It is therefore an object of this invention to provide improved gypsum molds for casting of ceramic articles that have enhanced absorption and purge rates.
It is another object of this invention to provide an inexpensive mold that retains strength for long life in addition to an increase in absorption and purge rate of absorbed water.
It is yet another object of this invention to provide an energy efficient mold for producing ceramic articles that does not require large amounts of energy to be expended to purge and dry the mold.
It is still another object of this invention to improve efficiency of production of ceramic articles by reducing the turn over time required to prepare molds for casting of another article.
It is another object of this invention to provide an improved mold formed from alpha calcined gypsum that absorbs water more quickly during pressure casting and reduces the time required to form the greenware.
The above-listed objects are met or exceeded by the present invention, which features a gypsum composition that produces an improved mold for casting ceramic articles. The mold has the improved ability to draw moisture from the article being cast, allowing it be produced faster, especially during pressure casting. Drying of the gypsum mold after use is also more efficient as it can be purged of the absorbed moisture more readily than a conventional gypsum mold. These improvements in absorbing and purging of water are done in a manner that does not compromise the strength of the mold or the efficiency of the casting process.
More specifically, the present invention provides a plaster composition for making gypsum molds for the manufacture of ceramicware. The plaster composition is comprised of a calcium sulfate hemihydrate, one or more of ammonium sulfate and potassium sulfate and one or more of ammonium L tartrate and potassium sodium tartrate. A weight ratio of from about 1:1 to about 10:1 of one or more of ammonium sulfate and potassium sulfate to one or more of ammonium L tartrate and potassium sodium tartrate must be maintained. The one or more of potassium sodium tartrate and ammonium L tartrate is present in an amount of from about 1 to about 10 pounds per ton of dry calcium sulfate hemihydrate, and the total of the ammonium sulfate, potassium sulfate, ammonium L tartrate and potassium sodium tartrate is from about 2 to about 35 pounds per ton of the dry hemihydrate.
Applicants have found that the combination of potassium sodium tartrate and potassium sulfate in a specified ratio produces a gypsum mold with a pore structure that facilitates enhanced purging of the water from the mold in preparation for its subsequent use. An improved mold made of alpha calcined gypsum also facilitates absorption of water during pressure casting. Ease in drying of the mold means that less energy is required in terms of pressure or heat to drive off the water that was absorbed by the mold""s capillaries during the process of casting the greenware. Less energy usage reduces the cost of the process. Ease in drying out the mold could, in some cases, also lead to a more efficient process because the molds could be more rapidly turned around and be ready for casting of a subsequent article.
The ease with which the water flows into and out of the capillaries of the mold is also evidenced by a greater turn over rate. Molds made from a traditional composition can be used to make an average of 80 articles before they become unusable. Life spans of 100-120 cycles have been achieved using the composition of the present invention. Increased mold life leads to reduced cost and a more efficient process.
Typical gypsum compositions utilize a higher water to plaster ratio for the purpose of increasing pore structure which, in turn, will enhance absorption and purge properties. The higher water to plaster ratio tends to lower mold life and strength. Applicants"" composition allows for increased absorption and purge properties at discernibly lower water/plaster ratios which provide higher strength and longer mold life. The addition of potassium sodium tartrate is generally known to reduce the strength of plaster compositions. However, the proportions of potassium sulfate and potassium sodium tartrate lead to a composition which maintains good strength while providing an improved pore structure that allows it to more efficiently absorb and purge out water from the mold when cast at a lower water to stucco ratio.
Yet another result of this composition is the improvement in the energy efficiency of the drying process and the pressure casting process. This efficiency is seen both during mold drying and, if the gypsum mold is formed from alpha calcined gypsum, during absorption of water from the greenware while pressure casting. If capillary flow is restricted, more energy must be expended to dry the mold for its next use. Molds may be dried by heat, pressure or a combination of the two. Regardless of which of these processes is used to dry the mold, the energy cost for the overall process can be reduced if temperature and pressure can be reduced. The composition and molds of the present invention allow temperature and/or pressure to be reduced while purging moisture from the mold, resulting in an energy efficient process. If pressure casting is used, lower pressure or less time in the pressurized environment is required where there is less resistance to capillary flow.
An improved mold may be made from a plaster slurry that is made of calcium sulfate hemihydrate, one or more of ammonium sulfate and potassium sulfate, one or more of ammonium L tartrate and potassium sodium tartrate and water in the same proportions as above. After the slurry is mixed, it is poured on a pattern to make a mold and allowed to set. When set, the pattern is removed from the mold then dried. A mold made in this manner is purged more quickly than molds made from prior art processes, which allows the user to reduce energy costs by drying with lower temperatures or pressures. Additional cast articles may also be produced for the same cost using the method of the present invention by speeding up the drying process and turning over the molds for re-use more quickly.
Processes which utilize this composition also manifest these advantages as well. In an improved method of making cast articles, cast articles are produced using a mold made from calcium sulfate hemihydrate, one or more of ammonium sulfate and potassium sulfate, one or more of ammonium L tartrate and potassium sodium tartrate and water. A weight ratio of from about 1:1 to about 10:1 of one or more of ammonium sulfate and potassium sulfate to one or more of ammonium L tartrate and potassium sodium tartrate may be used to make the mold. Articles are cast by pouring clay slip into the mold and allowing the slip to set. After removing the article from the mold, the mold is then purged to remove most of the absorbed water, then dried. Due to the increased purging rate, energy usage may be reduced, or the mold may be turned over more quickly to produce additional cast articles in the same time frame.
The present invention provides a plaster composition for making gypsum molds for the manufacture of ceramic articles which are more easily purged of absorbed water than prior art molds. Drying is a generic term for the removal of water from the mold, whereas purging is removing water by the application of pressure. In a preferred embodiment of the invention, the composition comprises a fluid mix of a calcium sulfate hemihydrate, water, potassium sulfate and potassium sodium tartrate.
The potassium sulfate and potassium sodium tartrate are present in a weight ratio of from about 1:1 to about 10:1. In a preferred embodiment, the weight ratio of potassium sulfate to potassium sodium tartrate is from about 2.5:1 to about 10:1. The potassium sodium tartrate is present in an amount of from about 1 to about 10 pounds per ton of plaster composition. The total of the potassium sulfate and potassium sodium tartrate is from about 2 to about 35 pounds per ton of dry calcium sulfate hemihydrate. When it is desired to use larger amounts of potassium sodium tartrate, a ratio of potassium sulfate at the lower end of the range of 2.5:1 to about 10:1 must be selected in order to keep the maximum amount of additives within the preferred limits. Although the discussion focuses on the preferred embodiment, it is understood that ammonium sulfate may be substituted for some or all of the potassium sulfate and ammonium L tartrate may be substituted for some or all of the potassium sodium tartrate.
A plaster composition starting material comprises calcium sulfate hemihydrate. This may be in either the alpha or beta form or any combination thereof. It is made by converting calcium sulfate dihydrate to calcium sulfate hemihydrate by the following reaction formula: 
This reaction can be carried out by calcination, or any other methods known in the art. Calcination utilizes heat to drive off some of the water of hydration under extremely controlled conditions. A small percentage of calcium sulfate hemihydrate may be completely removed of chemically bound water forming a calcium sulfate anhydrite impurity. The calcination process requires large amounts of energy to drive off water and must be carefully controlled to retain the correct water of hydration. It is also known to prepare the calcium sulfate hemihydrate using sulfuric acid at temperatures above 80xc2x0 C. These and other methods of reducing the water of hydration to obtain the hemihydrate are well known to those skilled in the art.
Calcium sulfate hemihydrate may produce at least two crystal forms. The alpha calcined gypsum forms less a circular crystals that pack tightly together, making a denser and stronger plaster. The crystal morphology allows water to flow easily between the crystals, requiring less water to form a flowable slurry. It is made by a continuous process whereby the calcium sulfate dihydrate is heated under pressure. More elongated crystals are characteristic of the beta calcined gypsum. This crystal structure results in a less dense product because the crystals are more loosely packed. The beta form also requires more water to fluidize the gypsum. If the calcining of the dihydrate is performed at ambient pressure, the beta form is obtained. Often it is advantageous to mix alpha and beta forms to produce a plaster with intermediate values of porosity and strength. Regardless of the form, alpha or beta, the set gypsum formed by the present invention has an improved ability to quickly purge the mold to dry it. The additives of the present invention provide increased purgability of molds, whether they are constructed from alpha-hemihydrate, beta-hemihydrate or mixtures thereof.
In order to create a gypsum mold, the calcium sulfate hemihydrate is mixed with water to form a slurry. Some of the water reacts with the calcium sulfate hemihydrate and anhydrite, if present, in the hydration process, essentially reversing the chemical formula given above. As the hemihydrate absorbs the water, crystals of the dihydrate start to form. These crystals grow and form an interlocking matrix of set gypsum crystals that has pores and capillaries in the crystal matrix. This matrix gives gypsum molds a high capacity for capillary action and allows water to be wicked away from the clay slip, forming greenware. The water is then either forced through and out of the mold or stored in the capillaries of the gypsum mold until the greenware is removed. The gypsum mold is then dried and/or purged of the water in preparation for the next use.
Addition of potassium sulfate and potassium sodium tartrate change the pore structure in the dihydrate as it forms. The resulting changed pore structure facilitates the removal of water either through purge and/or the use of heat. In addition, the changed pore structure allows for better absorption properties of gypsum formed at lower water to plaster ratios.
Amounts of the additives are critical. Potassium sulfate and potassium sodium tartrate may be dry mixed with the calcined gypsum or added directly to the mix water. If too much potassium sodium tartrate is added, the resulting plaster has reduced strength, and the mold does not hold up well to multiple uses. The potassium sodium tartrate, also known as Rochelle Salt, must be added in quantities of not more than 10 pounds per ton of calcium sulfate hemihydrate. Less than one pound of potassium sodium tartrate per ton of the hemihydrate is not as effective in increasing the purge rate of the mold.
The total amounts of the salts and the ratio of the potassium sulfate to the potassium sodium tartrate are also critical. Too much salt destroys the strength of the mold. No more than 35 pounds of additives per ton of calcium sulfate hemihydrate may be added to the plaster composition. The weight ratio of the potassium sulfate to the potassium sodium tartrate should not be less than 1:1, nor greater than 10:1. Before addition to the plaster composition, the additives should be ground until no more than 10% is retained on a 50 mesh sieve.
Ammonium L tartrate is chemically suitable for use in this composition in place of potassium sodium tartrate, however, it is not as readily available in quantities that would be necessary for a commercial process. Further, less pure forms of ammonium L tartrate have been found to cause a discoloration in the mold which usually renders it unacceptable to the ceramic manufacturer. Ammonium sulfate may be used in place of potassium sulfate.
The additive combination of the present invention cannot be used when the plaster composition contains alkaline additives such as Portland Cement and/or lime. The resulting pH of the slurry interacts with the potassium sulfate and potassium sodium tartrate additives in such a way that the mixture exhibits an extremely long set time.
Mixtures of potassium sulfate and/or potassium sodium tartrate have been known in the art to control mold expansion during the setting process. During the setting process, expansion occurs as crystal growth begins. In creating the gypsum mold, this is undesirable because it can lead to distortion in the shape of the mold. A commercial plaster mixture, #1 Pottery Plaster LE available from USG Corporation, contains a plaster mixture which includes 4 pounds per ton of potassium sulfate and 2 pounds per ton of potassium sodium tartrate. This mixture is used when it is necessary to reduce the amount of expansion that occurs as the dihydrate crystals form. Expansion in the gypsum mold can lead to distortion of the finished ceramic piece.
However, a significant development of the present invention is that some combinations of ratios of additives have been found to significantly increase the purge rate and absorption of gypsum molds when used to make cast articles. This blend of additives is previously unknown for this purpose. Further, in some preferred embodiments, higher ratios of potassium sulfate to potassium sodium tartrate are used compared to known commercially available plaster mixtures.
Other additives may be added where use of the mold requires particular properties. Sodium trimetaphosphate may be added to the composition to improve both the strength and the purge rate of the finished molds. It may be added at the rate of about 1 pound of sodium trimetaphosphate per ton of calcium sulfate hemihydrate to about 5 pounds per ton. Preferably, the additive is present in amounts from about 1 to about 3 pounds per ton. Polyethylene glycol is another optional additive. The addition of this material also gives slight improvements in the mold strength, and it prevents migration of other additives, such as potassium sodium tartrate, to the surface of the mold. Polyethylene glycol also acts as a lubricant, aiding in the release of the greenware from the mold. It may be added in amounts up to about 20 pounds per ton, preferably from about 0.5 to about 10 pounds per ton. Retarders known in the art, such as tartaric acid, proteinaceous retarders, citric acid or ammonium tartrate may be used to better control the setting process if desired. Reinforcing materials may be added where increase in strength of the mold is required. Except as previously noted, other additives known to those skilled in the art may be used in conjunction with this invention.